Alpha-olefin oligomers having unsaturated double bonds and about 4 to about 24 carbon atoms are useful substances, and have been extensively used as monomers for olefin-based polymers and comonomers for various high-molecular polymers as well as raw materials for plasticizers, surfactants or the like. The α-olefin oligomers have been generally produced by oligomerizing ethylene in a solvent in the presence of a Ziegler catalyst.
However, in the oligomerization reaction, there is such a tendency that high-molecular polymers by-produced are deposited onto a wall surface of a vapor phase portion located in an upper part of a reactor with the passage of time. If the oligomerization reaction is continued without removal of the high-molecular polymers deposited, gradual growth of the polymers occurs, and the grown polymers are finally dropped in a reaction solution owing to increase in their weight. The high-molecular polymers dropped and included in the reaction solution will cause clogging of an external circulating-type heat exchanger provided for removing the heat of reaction, failure of pump or the like. Once such disadvantages occur, it is required to stop the continuous operation of the reactor and conduct time-consuming removal of the polymers deposited therein.
In order to inhibit the deposition of polymers onto the wall surface of the vapor phase portion of the reactor during polymerization reactions, for example, upon production of acrylonitrile-styrene copolymer resins, styrene resins or polymethyl methacrylate, there is known the method in which a mixed solution of monomers as raw materials and a solvent is sprayed into an upper vapor phase space of the reactor to wet the wall surface of the vapor phase portion of the reactor and thereby reduce the amount of the polymers deposited thereonto (Japanese Patent Application Laid-open No. 9-77806). However, this publication fails to mention details of the spraying method, though it is only described that the solution to be sprayed is kept at a temperature of 0 to 60° C.
Also, there is known the technique in which a part of a solvent is atomized and supplied to a vapor phase portion of a reactor to increase a concentration of the solvent in the vapor phase portion for preventing the deposition of scales thereon (Japanese Patent Application Laid-open No. 49-86478). However, this technique is applicable only to limited polymers such as polyacrylonitrile.